Final answer:
CDR1 and CDR2 diversity is provided by V and J gene segments, while CDR3 diversity comes primarily from the recombinant arrangement of V, D, and J gene segments, including random nucleotide insertions.
Step-by-step explanation:
The gene segments that provide diversity to CDR1 and CDR2 are the variable (V) and joining (J) segments. The diversity for CDR3 comes mainly from the additional random insertion of nucleotides at the junctions of gene segments during the process of V(D)J recombination. Specifically, the genetic diversity for CDR3 in T-cell receptors (TCRs) is provided by the rearrangement of V, D, and J gene segments. The V, D, and J segments recombine to create the variable region of the TCR ß chain, whereas the V and J segments recombine to generate the variable region of the TCR α chain. The D segment is unique to the TCR ß chain and the heavy chain of BCRs, contributing to even greater diversity of CDR3.
During T-cell maturation in the thymus, thousands of different V, D, and J gene segment combinations are created, providing a vast repertoire of T-cells each capable of recognizing a unique antigen via its TCR. The genetic recombination of these segments is a random process ensuring that each T-cell has a unique receptor specificity. This molecular mechanism is crucial for the adaptive immune system's ability to recognize and respond to a wide variety of microbial antigens.